The present application is related to metallic pastes. Metallic pastes have a number of uses, particularly in the field of printed circuitry. Metallic pastes can be printed (screen printed, inkjet printed, etc.) onto printed circuit boards, ceramic materials, three dimensional circuitry (e.g. cell phone antennas) and anywhere else where a conductive trace is needed. The most commonly used metallic paste is silver metallic paste. Palladium metallic paste is also common.
Both silver and palladium metallic pastes are expensive. One possible way of reducing the cost of using such pastes would be to use some other metallic paste and plating onto that metallic paste. One possible inexpensive metallic paste would be an aluminum paste. However, aluminum pastes cannot be directly plated with electrolytic processes because of the heavy, insulating oxide layers on the surface of aluminum particles. Furthermore, aluminum pastes are delicate and easily attacked, overly etched or destroyed during plating processes.
Electroless and electrolytic plating of conductive metals onto metallic pastes, such as silver pastes, are typically carried out using a non-etch soak clean step, an acid etch step, an alkaline zincate step and an electroless or electrolytic plating step. The metals are sufficiently conductive and can be directly electroplated or electrolessly plated after the zincate step using standard processes.
A metallic paste such as the aluminum paste discussed above is not designed or typically used for subsequent plating. Such a paste requires special processes to perform electroless or electrolytic plating onto it. This is because a metallic paste, such as aluminum, has heavy, insulating oxide layers on the surface of the aluminum particles. Furthermore, aluminum pastes are delicate and easily attacked, overly etched or destroyed during plating processes.
One possible application for a plated aluminum paste is in the manufacture of solar cells (also called a photovoltaic (PV) cell). A photovoltaic cell is an electrical device that converts the energy of light into electricity by the photovoltaic effect. It is a form of photoelectric cell (in that its electrical characteristics—e.g. current, voltage, or resistance—vary when light is incident upon it) which, when exposed to light, can generate and support an electric current without being attached to any external voltage source.
A typical silicon PV cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorus-doped (N-type) silicon on top of a thicker layer of boron-doped (P-type) silicon. An electrical field is created near the top surface of the cell where these two materials are in contact, called the P-N junction. When sunlight strikes the surface of a PV cell, this electrical field provides momentum and direction to light-stimulated electrons, resulting in a flow of current when the solar cell is connected to an electrical load
The process of fabricating conventional single—and polycrystalline silicon PV cells begins with very pure semiconductor-grade polysilicon—a material processed from quartz and used extensively throughout the electronics industry. The polysilicon is then heated to melting temperature, and trace amounts of boron are added to the melt to create a P-type semiconductor material. Next, an ingot, or block of silicon is formed, commonly using one of two methods: 1) by growing a pure crystalline silicon ingot from a seed crystal drawn from the molten polysilicon or 2) by casting the molten polysilicon in a block, creating a polycrystalline silicon material. Individual wafers are then sliced from the ingots using wire saws and then subjected to a surface etching process. After the wafers are cleaned, they are placed in a phosphorus diffusion furnace, creating a thin N-type semiconductor layer around the entire outer surface of the cell. Next, an anti-reflective coating is applied to the top surface of the cell.
An aluminized conductive material is deposited on the back (positive) surface of each cell, restoring the P-type properties of the back surface by displacing the diffused phosphorus layer. The aluminized conductive material is sometimes applied by screen-printing a metal paste, such as an aluminum paste.
Electrical contacts are also imprinted on the top (negative) surface of the cell. The grid-like metal contact made up of fine “fingers” and larger “bus bars” are typically screen-printed onto the top surface. This is typically done using a silver paste. As discussed above, silver pastes are very expensive. One step towards reducing the cost of PV cells is to print a finer silver grid and improve the conductivity of it by topcoating with various, less costly electroplated coatings such as nickel, copper, tin and various combinations of each. The plating of silver pastes is readily achieved by immersing the screen printed and fired wafers into a plating solution with the application of direct current or LIP plating. The metals are then deposited directly onto the silver paste without requiring any pretreatments due to the high conductivity and very thin oxide on the silver particles. As discussed above, a possible way of reducing the cost of PV cells would be to avoid the use of a silver paste by utilizing a plated aluminum paste.